Schedule changes & reminders:
*Dr. Majewska will not lecture on Wed March 16th;
NO CLASS THIS WEDNESDAY
*Next Mon March 21st: lecture on synaptogenesis
*Next Wed March 23rd: Dr. Robert Freeman lecturing
on trophic factors and programmed cell death (as scheduled)
*Exam 2 on Mon March 28th; old exam posted on BB
Refinement of synaptic connections—
1. Muscle activity correlates w/ timing of synaptic refinement
at the NMJ [TTX cuff = excess synapses vs. stimulating
electrode= rapid elimination]
2. Distance between contacts regulates synapse elimination
[shorter distance= more elimination]
3. Sensory experience regulates synaptic refinement and
sensory coding properties [or a neuron’s response to a
specific stimulus]
e.g. frequency tuning curve (plot of sound freq vs.
intensity at which freq evokes response)= a measure of
afferent innervation in the auditory pathway (IC)
Repeated broadband stimulus in young rodents inhibits
synaptic refinement
Clicks cause many auditory nerves to fire simultaneously
IC neurons less selective in response to various sound levels
 at which neurons respond
Broadened frequency curves remain throughout development
Limited visual experience leads to limited neuronal responses
Neuronal programmed cell death (apoptosis) during
development: a normal part of maturation required to
reduce redundancy
The four main stages of neural development
Developmental neuronal death is apoptotic; injury may be necrotic
Apoptosis triggers endonucleases to digest DNA, & proteases to
cleave proteins
Cell Death in the SNB: regulated by testosterone levels
Embryonic kitten retina has apoptotic neurons
DNA fragmentation patterns can distinguish apoptosis vs. necrosis
TUNEL: TdT dUTP nick end labeling (of DNA fragments)
TUNEL labeling of apoptotic neurons
The stages of neuronal death
caspases
activate
flippase
altered
reduction
of O2 to
O2 -
Many brain neural progenitors die just after S phase in late
development
ISEL labeling (Klenow, 5’ends); TUNEL labeling (TdT, 3’ends)
Neuronal survival factors have many sources
Adding or removing limb bud influences DRG/motor neuron
survival
Neuron # determined by proliferation, differentiation, & survival
signals from target
Limb bud removal experiment: less target = more DRG death
The majority of frog motor neurons die during normal development
Approximately half of all types of neurons die by maturity
-rat RGCs: 50% die
-chick ciliary ganglion
neurons: 50% die
-mouse cortex: 20-50%
neurons die
Differential DRG cell death along body axis correlates w/ target
size
Trophic factors derive from targets, afferents, neighboring
somata, blood, & glia
Removing the cochlea causes neuron death only until P9 (gerbils)
*early deafferentation
causes death b/c of
constitutive proapoptotic gene expression;
at P9, anti-apoptotic
genes turn on
Removing chick cochlea during development also causes
death (in the NM)
Inverse relationship between afferent target neuron death and
tissue generation
Neuronal survival from afferents, neighbors, glia, blood, or target
cells
Hormonal regulation of neuron death vs. survival
SNB (spinal nucleus of the bulbocavernosus) motor neurons
innervate perineal muscles required for male mating; are
nearly absent in females
Tumor cell-supplied neuronal survival factor: a soluble substance
secreted by target
The neuronal survival factor is a soluble protein (in venom and
salivary gland)
cultured sympathetic
neurons
*venom enzyme
digests DNA/RNA
Neurotrophins & their receptors are essential for neuronal survival
Neuronal death is dependent on protein synthesis
*translation or
transcription
inhibitors
Motoneurons & DRG neurons can be rescued in vivo via inhibiting
protein synthesis
Motor neuron death is activity-dependent & blocked by curare
(ACh antagonist)
Increased MN
survival: more
MN synapses on
muscle, more
target-derived
survival signaling
*TTX: blocks Na+
channels/action
potential
Hermaphrodite vulval neural precursors are protected from death